smaller in Scorpio than in Limulus--in fact is minute, as
it is in all the terrestrial Arachnida which suck the juices of either
animals or plants. In both, the alimentary canal takes a straight
course from the pharynx (which bends under it downwards and backwards
towards the mouth in Limulus) to the anus, and is a simple, narrow,
cylindrical tube (fig. 33). The only point in which the gut of Limulus
resembles that of Scorpio rather than that of any of the Crustacea, is
in possessing more than a single pair of ducts or lateral outgrowths
connected with ramified gastric glands or gastric caeca. Limulus has
two pairs of these, Scorpio as many as six pairs. The Crustacea never
have more than one pair. The minute microscopic structure of the
gastric glands in the two animals is practically identical. The
functions of these gastric diverticula have never been carefully
investigated. It is very probable that in Scorpio they do not serve
merely to secrete a digestive fluid (shown in other Arthropoda to
resemble the pancreatic fluid), but that they also become distended by
the juices of the prey sucked in by the scorpion--as certainly must
occur in the case of the simple unbranched gastric caeca of the
spiders.

The most important difference which exists between the structure of
Limulus and that of Scorpio is found in the hinder region of the
alimentary canal. Scorpio is here provided with a single or double
pair of renal excretory tubes, which have been identified by earlier
authors with the Malpighian tubes of the Hexapod and Myriapod insects.
Limulus is devoid of any such tubes. We shall revert to this subject
below.

[Illustration: FIG. 26.

A, Diagram of a retinula of the central eye of a scorpion consisting
of five retina-cells (ret), with adherent branched pigment cells
(pig).
B, Rhabdom of the same, consisting of five confluent rhabdomeres.
C, Transverse section of the rhabdom of a retinula of the scorpion's
central eye, showing its five constituent rhabdomeres as rays of a
star.
D, Transverse section of a retinula of the lateral eye of Limulus,
showing ten retinula cells (ret), each bearing a rhabdomere (rhab).

(After Lankester.)]

[Illustration: FIG. 27.--Diagram showing the position of the coxal
glands of a scorpion, _Buthus australis_, Lin., in relation to the
legs, diaphragm (entosternal flap), and the